468. Validate IP Address 

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class Solution {
    public String validIPAddress(String queryIP) {
        if (isIPv4(queryIP)) {
            return "IPv4";
        } else if (isIPv6(queryIP)) {
            return "IPv6";
        } else {
            return "Neither";
        }
    }

    private boolean isIPv4(String queryIP) {
        String[] strs = queryIP.split("\\.", -1);
        if (strs.length != 4) {
            return false;
        }
        for (String str : strs) {
            if (str.isEmpty() || (str.charAt(0) == '0' && str.length() > 1) || str.length() > 3) { // 此处的长度限制能够避免后续转换数字时越界
                return false;
            }
            int num = 0;
            for (int i = 0; i < str.length(); i++) {
                if (!Character.isDigit(str.charAt(i))) {
                    return false;
                }
                num = num * 10 + (str.charAt(i) - '0');
            }
            if (num > 255) {
                return false;
            }
        }

        return true;
    }

    private boolean isIPv6(String queryIP) {
        String[] strs = queryIP.split(":", -1);
        if (strs.length != 8) {
            return false;
        }
        for (String str : strs) {
            if (str.isEmpty() || str.length() > 4) {
                return false;
            }
            for (int i = 0; i < str.length(); i++) {
                char c = str.charAt(i);
                if (!isHexChar(c)) {
                    return false;
                }
            }
        }

        return true;
    }

    private boolean isHexChar(char c) {
        return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F');
    }
}

注意单个参数的 String#split 方法默认未包含末尾的空字符串，若需要包含末尾的空字符串，则需要显式指定第二个参数为负数。

References

468. Validate IP Address
Why does “split” on an empty string return a non-empty array?